Tuberculosis (TB) is the leading cause of death in AIDS patients worldwide. Although the use of highly active antiretroviral therapy (HAART) during TB treatment in HIV-1/Mycobaterium tuberculosis (MTb) co-infected patients is associated with reduced mortality, it can also result in life threatening "paradoxical reactions" (PR), which occur in 7-36% of co-infected patients treated for both diseases. PR occurs after initial clinical TB improvement and within approximately 4-6 weeks after HAART initiation. Very little is known about the origin and mechanisms behind PR, except that it is associated with increased CD4+ T cell counts and the exacerbation of tuberculin-specific Th1 responses in peripheral blood. Our hypothesis, to be tested in this proposal, is that the mechanism underlying PR in HIV-1/TB co-infection occurs due to an unbalanced reconstitution of immune effector responses resulting in exaggerated CD4+ {and CD8+}T lymphocyte activities to MTb antigens. We hypothesize that these exaggerated T cell responses develop in PR+ patients, but not in PR- patients, due to a more robust rise of MTb-specific T cells and/or decreased reconstitution of immunosuppressive regulatory T cells (Tregs) within the CD4+ T cell compartment. Here, we propose to test this hypothesis by investigating peripheral blood CD4+ (memory, naive, Tregs) and {CD8+ (memory, naive)} T cell subsets and their function in immunosuppressed TB/AIDS patients on dual therapy from an ongoing clinical trial in Cambodia. This trial, called CAMELIA (CAMbodian Early vs. Late Introduction of Antiretrovirals), is designed to determine the optimal timing of HAART initiation in co-infected AIDS patients on TB therapy. {Nesting these studies in the CAMELIA gives us a unique time-limited opportunity} to (i) define the immunological mechanisms underlying PR;(ii) define the mechanisms of reconstitution of T cell functions in TB/HIV-1 co-infection;and (iii) to associate these T cell responses with clinical and laboratory markers found in PR patients in the CAMELIA trial.} To accomplish these goals, we will prospectively analyze CD4+ and CD8+ T cell subsets and their proliferative capacity and cytokine responses using several methods, including intracellular cytokine staining (ICS), proliferation analysis and ELISA, in response to a number of MTb and HIV- 1-specific stimuli. We also hypothesize that we will identify specific anti-TB antibody responses in PR+ patients by comparing responses prior to and after HAART initiation, as well as during PR. We anticipate that these studies will elucidate the origin and mechanisms underlying PR, and will identify key features of CD4+ {and CD8+ } T cell restoration during HAART and TB therapy. We also expect that we will identify markers and new anti-TB antibody responses that are predictive of the emergence of PR. Finally, we anticipate that these studies will give us fundamental insights into the immune response to TB and HIV-1 and, thus, to new therapeutic approaches to TB and AIDS co-infection. PUBLIC HEALTH RELEVANCE: Tuberculosis (TB) is the largest cause of death in the setting of AIDS, particularly in Asia and Africa, and has claimed an estimated one third to one half of the 30 million people who have from AIDS-related causes to date. Unfortunately, simultaneous treatment of the two diseases is complicated by a phenomenon known as the `paradoxical reaction'where the patient deteriorates with symptoms of progressive TB after symptoms had improved on drug therapy. Intriguingly, the paradoxical reaction appears to be an immune-mediated phenomenon since both the TB and HIV-1 infections are responding to therapy when it occurs. Thus, the paradoxical reaction provides a unique opportunity to study exaggerated immune responses to TB and AIDS and, by doing so, to gain a fundamental understanding into the basic immune response to both pathogens. In addition, another goal of this proposal is to better understand the basic immune response involved in the paradoxical reaction so that we may be better able to predict who may be susceptible to it, as well as devise better treatment strategies for when it occurs. We will conduct these studies in the context of an ongoing clinical trial designed to determine the optimal timing of TB and AIDS co-therapy in Cambodia.